Blasting compositions consisting essentially of particulate ammonium nitrate, coated with a small proportion of oil or other fuel, are widely used for blasting purposes because of their low cost. Fertilizer grade ammonium nitrate prills, coated with about 6% of diesel oil or fuel oil is widely used under the general title ANFO (ammonium nitrate fuel oil). These compositions, however, have certain limitations. They are low in water resistance and usually cannot be used in water filled boreholes or in other wet places, requiring that they be packaged in plastic bags to keep them dry. Then their bulk density is usually much less than that of water so they will not sink in a water filled hole without some weighty material being added. They are also hygroscopic so that they will cake and become inert in damp air, even though no actual water is present. Moreover, their bulk blasting energy (Energy per unit of volume) is quite low, requiring that larger diameter bore-holes be bored than would be required for a more powerful explosive. This of course requires larger drills, more power to do the drilling, and adds to blasting costs. Holes as large as 15 inches in diameter are frequently used for ANFO blasting. ANFO has a slower detonation velocity than some other blasting compositions which is an advantage where a heaving action rather than shattering is desired. Some attempts have been made to increase the density of ANFO, as by crushing the granules, or packing them under compression in strong bags but at best a bulk density of about 1 gram/cc. is the maximum that can be obtained, and this under difficulties. Bagged ANFO even of density above 1.0 still will not sink readily in water filled boreholes. In come cases fuels such as ethylene glycol and/or powdered aluminum have been added to increase the density but these add to costs; particulate ferrosilicon or ferrophosphorus, similarly used, are costly too.
Because of the above disadvantages, slurry explosives have come to replace ANFO for wet conditions in many cases because they can be made more dense and they have some temporary water resistance. These usually consist of (1) a thickened or gelled aqueous solution of ammonium nitrate, (2) an added fuel which may comprise fuel oil, liquids such as alcohols, etc., or solid particulate materials such as sulfur, coal, gilsonite, and/or aluminum particles, and (3) some undissolved oxidizer, usually ammonium nitrate, in suspension in the liquid. The slurries may be cross-linked with chemical agents to stabilize their gelled condition and they may be pumped directly into boreholes where, under favorable conditions, they resist water penetration temporarily. They may have a density as high as 1.2 gm./cc, or even higher so they will sink in water. However, they usually contain about 15% or so of water which reduce their weight strength (energy per unit weight) below that of ANFO, usually to about 60 to 80% that of ANFO unless high energy (and costly) fuels such as paint grade aluminum or the like are included.
Another disadvantage of slurries is that at their natural densities they usually will not detonate reliably unless they are sensitized, e.g. aerated by inclusion of finely dispersed gas, either as minute bubbles or enclosed within fine hollow particles such as styrene beads, microspheres of glass filled with air, or the like. Gas bubbles are often generated in situ in the gel, as by decomposing a gas-generated material such as hydrogen peroxide or a nitrite or carbonate salt in the slurry, often requiring close pH control, along with a catalyst to promote gas formation. Although this is effective in some cases, where the explosive is loaded into deep boreholes, with a high hydrostatic load of explosive above, the bottom part of the charge often is so much compressed that the aeration is ineffective and the bottom part of the charge may fail to detonate, even though part of the charge above shoots satisfactorily. In such cases, much more gasing agent or aeration, or the equivalent, must be used at the bottom than at the top and this causes difficulties. The slurry tends to rise out of the water filled hole as it is poured or pumped into place. Highly trained personnel are often required to manage slurry satisfactorily, e.g. by pulling the filling hose up at a carefully controlled rate as the charge is pumped into place, aside from the care required in preparing the explosive mixture. Changes in formulation, from bottom to top of the hole are often required and this requires skilled manipulations. Packaged slurry products often have densities around 1.1 g./cc. and high detonation velocities.
It has been suggested in the prior art that ANFO may be modified by combining other materials with it, to overcome some of its deficiencies. Thus in Clay U.S. Pat. No. 4,111,727, it is suggested that a water-in-oil emulsion be used to partially fill the interstices between the solid granules; however, in some cases this is not as highly water resistant as may be needed. Water-in-oil emulsions to be used as explosives per se have been described by Bluhm in U.S. Pat. No. 3,447,978, also in British Pat. No. 1,306,546. Egly et al, in U.S. Pat. No. 3,161,551 has suggested use of an emulsion to fill around ammonium nitrate prills but made no provision for sensitivity and such compositions usually fail to detonate. In a copending application, Ser. No. 936,926, filed Aug. 25, 1978, the present applicant has suggested that an emulsion be used to fill all the voids and that gas-containing hollow microspheres be included to impart sensitivity.
According to the present invention, a filler material is also used but it is one which can be better controlled as to sensitivity. It contributes substantial energy and does not adversely affect the solid particles of ammonium nitrate which make up the main bulk of the charge. It is based on the discovery that certain non-aqueous types of slurry can partially fill the interstices and pores in and between the granules without softening or partly dissolving the granules themselves. This maintains good consistency as well as good sensitivity of the combined solid granules and slurry. The slurries used for this purpose, according to my invention, are based on organic liquid solutions of ammonium and/or calcium nitrate, with or without some other salts, wherein the solvent is a polar liquid, a low molecular weight organic liquid compatible with water and having reasonably good solvency for the nitrates, along with other powerful oxidizer salts, selected from the group which consists of methanol, ethylene glycol, formamide and ethanol. In some cases isopropanol and/or propylene glycol can be used as part of the solvent. As much salt as is practical should be dissolved in the solvent for maximum benefit.
By the present invention, major disadvantages of the prior art reinforced ANFO compositions can be avoided while still retaining most of their advantages. Objectives and advantages of the present invention and of products made according to it are listed as follows:
(1) A blasting agent of superior bulk energy is made available, still based largely on the use of relatively inexpensive fertilizer grade ammonium nitrate. Compared to ANFO with a relative bulk strength of 1.0, the bulk energy of the improved compositions may be increased and may be controlled over a rather wide range from 1.0 to as much as 1.5. The weight energy (blasting energy per unit of weight) can be at least substantially equal to that of ANFO, thus permitting equivalent blasting with smaller and less expensive boreholes.
(2) Where it is desired to have a detonation velocity lower than that of some of the older conventional blasting agents, for lower fragmentation and more heaving action, this improved composition can have definite advantages. With them, lower velocities may be obtained without substantial loss of power. In situations where extra energy is desirable, and where fuel particles such as coarsely divided aluminum are added to obtain the extra energy the longer reaction time thus provided also can result in fuller combustion of the fuel particles and therefore better utilization of such fuels. This applies not only to aluminum fuels but to other particulate fuels as well.
(3) According to the present invention, the blasting agent can be prepared at relatively low cost, compared to other agents of similar energy. With substantially higher density then ANFO, and consequent higher bulk energy, it can be made denser than water so as to sink readily into water filled boreholes. For this purpose, it may be bagged in water-proof containers which usually is considered necessary in such cases. Although some success has been obtained in increasing density of normal ANFO, e.g. by adding fines, etc., the density thus obtainable is usually not sufficient to have the material sink readily into water. Rapid sinking can be obtained with the present improved products.
(4) The agents of this invention can readily be delivered, at variable and controlled density, from modified or even conventional ANFO delivery trucks.
(5) For very deep holes, where extra blasting strength may be needed at the bottom, dependable blasting compositions of higher density, and consequent greater bulk energy, can be delivered to the bottom of the hole while those of lesser density and lower cost can be used towards the top where less energy is required.
(6) The agents of the present invention can be made in most parts of the world from locally available materials.
(7) The compositions are quite insensitive to inadvertent or unintentional detonation due to external causes over a wide temperature range.
(8) The agents are also insensitive to sparks, shocks in handling, rifle bullets or blasting caps.
(9) No complicated special equipment is needed, either for preparation or for delivery at borehole sites.
(10) These compositions can readily be cleaned up and spills removed by simple washing with water.
(11) They can be safely stored at a plant or in a delivery truck. They are safe and stable over long periods of time and under various environmental conditions.
(12) At the same time, they are reliably detonable by standard booster type detonators.
It has been known in the past to use organic liquids such as those mentioned above as fuel in slurries per se but in the present case, they are used also as primary solvents. Thus advantage is taken of their fuel values while avoiding the disadvantages of aqueous slurry in combination with ANFO. A good example of prior art use of these organic liquids in slurries is found in Funk et al U.S. Pat. No. 3,765,967, where they are described as being useful in small diameter charges, provided they can be detonated. To insure detonation, this reference suggests incorporation of self-explosives and contemplates use of perchlorates in the oxidizers. It is preferable to avoid use of such materials for safety reasons, not to mention expense; they are not needed in the present invention.